spi: dt-bindings: add mising description type to reg property

Added missing description type.

Fixes warning:
Documentation/devicetree/bindings/spi/spi-sunplus-sp7021.yaml: properties:reg:items: 'anyOf' conditional failed

Fixes: a708078e ("spi: Add Sunplus SP7021 schema")

Signed-off-by: David Heidelberg <david@ixit.cz>
Link: https://lore.kernel.org/r/20220202104715.27839-1-david@ixit.cz


Signed-off-by: Mark Brown <broonie@kernel.org>